Ferroelectric materials have been widely studied and applied to the application of non-volatile random access memory (NVRAM), particularly over the last decade. Pb(ZrTi)O3 (PZT) has been one of the leading materials, which exhibits a high value of remnant polarization; it also can be fabricated at relatively low crystallization temperatures. Fatigue (defined as loss of polarization with repeated switching of the field) and imprint are the principle factors limiting commercialization of PZT based NVRAMs. Presently, PZT thin films do not show fatigue resistance above 106 switching cycles on mental electrodes such as Pt.
To overcome the problem of fatigue, a number of attempts have been made to fabricate PZT on oxide electrodes (e.g., RuO2, (LaSr)CoO3, LaNiO3, etc.). These have been able to obtain fatigue resistance up to 1010 switching cycles. This fabrication, however, presents another problem. The integration of oxide electrodes in Si technology complicates the design rules for various commonly-used interconnections.
C. A. Paz de Araujo, et al., Fatigue-Free Ferroelectric Capacitors with Platinum Electrodes, Nature, Vol. 374, p. 627 (1995) introduced a new application of so-called Bi-layered compounds, which showed fatigue resistant up to 1012 switching cycles. Bi-layered compounds were first discovered by B. Aurivillius, Ark. Kemi, Vol. 1, p. 463. C. A. Paz de Araujo, et al., studied Aurivillius-layered ferroelectrics, in particular SrBi2Ta2O9 (SBT), in thin film form for its possible application to memory devices because of having a low leakage current density (˜10−7 A/cm2), low operating voltage, fast switching, and fatigue endurance of up to 1012 switching cycles. Unfortunately, however, some of the intrinsic fundamental material problems associated with SBT include: (1) a low remnant polarization (Pr=10 μC/cm2); (2) a relatively high crystallization temperature (˜750° C.); (3) a strong anisotropy characteristics; and (4) fabrication difficulty for thin films on metal electrodes along the polarization axis (a–b plane).
Many attempts have been made to overcome these drawbacks by doping different concentrations of cations at Sr- or Ta-sites, or by growing off-stoichiometric SBT with excess Bi concentration. SrBi2Nb2O9 (SBN) has the same crystal structure with higher curie temperature (˜440° C.) than for SBT (335° C.). The displacement of Nb ions inside the octahedral NbO6 is larger than Ta inside TaO6 octahedra, which is expected to show a higher value of remnant polarization. The biaxial strain of SBN is quite small <0.002, and the polarizability of niobate systems is in most cases larger than that of tantalite systems.
K. Watanabe et al., Spin-coated Ferroelectric SrBi2Nb2O9 Thin Films, Applied Physics Letters, Vol. 73, No. 1, p. 126 (1998), reported a remnant polarization of 12.5 μC/cm2 in the case of randomly-oriented SBN thin films.
In addition, two reports have studied textured and epitaxial SBN thin films, namely: J. Lettieri, et al., Epitaxial Growth of SrBi2Nb2O9 on (110) SrTiO3 and the Establishment of a Lower Bound on the Spontaneous Polarization of SrBi2Nb2O9, Applied Physics Letters, Vol. 77, No. 19, p. 3090 (2000) and J. Lettieri, et al., Epitaxial Growth of Non-c-oriented SrBi2Nb2O9 on (111) SrTiO3, Applied Physics Letters, Vol. 76, No. 20, p. 2937 (2000). According to these references, an SBN thin film is able to achieve a remnant polarization of ˜11.4 μC/cm2 on a Pt-coated silicon substrate with a SrRuO3 buffer layer. In addition, a higher remnant polarization of 15.7 μC/cm2 of epitaxial SBN thin films was obtained at an extremely high processing temperature of 877° C. on an oxide SrRuO3/SrTiO3 electrode.
A stable, thin film exhibiting greater remnant polarization is desired. More specifically, a remnant polarization of greater than 25 μC/cm2 is desired without the extremely high processing temperatures required by the prior art.